User Manual

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Discover
At this step, the LSR will send Hello messages periodically to adjacent LSRs, notifying them about its
existence, in order to establish a session. In the basic discover mechanism, LSR will discover its LDP peers
automatically via this process without manual configuration. There are two discover mechanisms:
 Basic Discover Mechanism
The Basic Discover Mechanism is used to discover local LDP peers – LSRs directly connected via the link
layer, and create a local LDP session. In this mode, the LSR will send LDP Link Hello messages periodically
via UDP messages to the multicast address marked as “all routers in the subnet”.
LDP Link Hello messages carry the LDP ID of the interface and other related information. If the LSR receives
a LDP Hello Message at an interface, it means that there is a LDP peer at this interface (Link Layer).
 Extended Discover Mechanism
The extended discover mechanism is used to discover remote LDP peers – LSRs not directly connected via
the link layer, and created remote LDP sessions. In this mode, the LSR will send LDP Targeted Hello
messages periodically to the specified IP address via UDP messages.
LDP Targeted Hello messages carry the LDP ID of the interface and other related information. If the LSR
receives a LDP Targeted Message at an interface, it means that there is a LDP peer at Network Layer.
Establish and Maintain the Session
After discovering a LDP peer, LSR will began to establish the session in two steps:
 Establish the transmission layer connection, that is, a TCP connection between LSRs;
 Initiate the session between the LSRs, negotiate all concerning parameters, such as the LDP
version, the label advertisement mode, the timer value, the label space. After the negotiation
succeeds, the session is established between the LSRs.
The session will be maintained by Keepalive messages after established.
Create LSP
The process of creating LSP is mapping FEC and labels and advertising the maps to the adjacent LSRs along
the LSP, which is realized via LDP. Take DoD mode as the example, the main steps are as follows:
（1） When the network routes change, if an edge router finds out a new destination address in its route
table which belongs to none of the existing FECs, it needs to create a new FEC for this destination
address. The edge LSR determines the route for the FEC, initiates a label request message to its
downstream LSR, and specifies for which FEC this label request is.
（2） The downstream receiving the label request message will save this message, finds the
corresponding FEC next-hop according to the local route table and then sends a label request
message to its downstream.
（3） When the label request message reaches the destination router or the egress router of the MPLS
network, if the router has available labels, and judges the label request messages as legal, it will
distribute a label for the FEC, and send a label mapping message containing the label information to
its upstream;
（4） The LSR receiving the label mapping message will check the state of label request messages saved
locally. If there is a corresponding label request message of a FEC label mapping message in the
data base, LSR will distribute a label for the FEC, and add a new entry in its LFIB, and then send the
label mapping information to its upstream.
（5） When the ingress LSR receives a label mapping message, it also should add a corresponding entry
in its LFIB, and thus finish the creation of LSP.